Seth Rogen and Rose Byrne start a war with the frat house next door in "Neighbors," a chilling look at aging that turns anxiety into decent gags.

Inside every Apatow and Apatow-related product, buried under a metric ton of dirty jokes and weed talk, lies deep anxiety over aging, over mediocrity, even over mortality. Apatow’s own “This is 40” claimed itself as the semi-sequel to “Knocked Up,” but the real spiritual follow-up is “Neighbors.” Here, Seth Rogen’s Mac not only has an infant but a suburban home, a soul-sucking desk job and no energy for nightlife. He’s living the nightmare Rogen’s character likely had in “Knocked Up,” and when a raucous frat, led by a terminally shirtless Zac Efron, moves next door, Mac can only seem like the boring, uptight enemy — the bleak future of hangovers and container stores that awaits them.

The whole of “Neighbors” is fueled by this unease. Attempts to seem cool backfire, and a call to the cops during a late weeknight kegger leads to an all-out war between the two factions. Mac isn’t in it alone; he’s joined, each and every step, by his wife Kelly (Rose Byrne), who refuses to fill the stereotype of the nagging harridan. Good and noble intensions lead to rash, sometimes successful, sometimes disastrous, always at least very funny set pieces. They’re both man-children and not very smart, going stir crazy in the house in which they dropped all their money, in the same anonymous college town they’ve lived since they were the age of the douches next door.

Kelly’s still a guy’s invention: the wife who joins in on the fun, and is just as freaked out as the guy is when the breast pump breaks and she requires milking. But for a female to have fun without being a ballbreaker is big in the expanded Apatow universe, and Byrne — a serious actress who revealed she has surprising comic elan in “Get Him to the Greek” and “Bridesmaids” — frequently steals the picture, including from her giggling, laidback co-star, who only gets amped up when wielding a dildo as a weapon.

Efron does some stealing too, if only because he’s in the role he was born to play. Efron’s Teddy is the kind of guy who went to school only to extend and amplify the partying he did in high school. He’s not intelligent, though some of his “brothers” — including his bestie, played by Dave Franco — are, and he knows that his days are numbered. It’s a natural fit for Efron, who oozes a classic Rob Lowe vibe, meaning he’ll be even funnier when he winds up on an NBC sitcom in his 40s.

“Neighbors” doesn’t get too heavy with any of this; it’s as terrified of maturity as its characters. It just knows how to translate them into gags, including a bone-chilling off-hand bit where one fratboy (Jerrod Carmichael) clearly has no idea who Garfield is. It’s an embodiment of immaturity and an exploration of it, and if it weren’t as funny or as sharp as it is — and it's directed by "Forgetting Sarah Marshall"'s very precise Nicholas Stoller — that might have been a problem.